In this Special Issue of New Phytologist on Plant Evolutionary Radiations, Koenen et al. present compelling evidence for recent evolutionary radiations in rain forest mahoganies in the Amazon. The mahogany family (Meliaceae) comprises 700 species of deciduous and evergreen trees in seasonally dry and wet forests across the tropics. Koenen et al. reconstruct a phylogeny for the family and estimate spatiotemporal diversification trajectories in one of the first studies to compare diversification patterns across lineages spanning dry and wet tropical forests. They show that the majority of mahogany species diversity in tropical rainforests is recent and the result of higher speciation rates compared to non-rainforest lineages.

While it is apparent that other species-rich Amazonian tree lineages also diversified rapidly in the late Miocene and Pliocene, this study provides much more convincing evidence that these rainforest clades, viewed in the context of a global analysis of the whole family, do constitute radiations subtended by species diversification rate increases. It is striking that these rain forest radiations are ecologically, structurally and functionally convergent and happened in the same place at the same time in different lineages. If we believe the age estimates in this study, most of the species in these radiations evolved during the Pleistocene, an idea that has been the focus of much debate surrounding Amazonian Pleistocene refuge theory, but which has not been demonstrated before in macroevolutionary studies of Amazonian trees.

Figure 2 from the paper (Koenen et al.) Phylorate plot with branches colored by speciation rate (lineages/Myr) as indicated by the scale bar, representing a summary of the full post-burn-in Markov chain Monte Carlo (MCMC) sample of the Bayesian analysis of macro-evolutionary mixtures (Bamm) analysis. Red circles indicate the positions of regime shifts in the maximum a posteriori (MAP) configuration (the MAP configuration is also shown in the upper left panel of Supporting Information Fig. S6). Numbers beneath the shifts indicate the marginal probability of a shift occurring along that branch. The tree displayed here is the maximum clade credibility (MCC) tree of Scenario 2.

The evolution of tropical rain forest hyperdiversity remains poorly understood. On the one hand rain forests have been viewed as museums where diversity accumulated at a constant slow rate with low extinction throughout the Cenozoic. Others have suggested that most rain forest diversity is much more recent, invoking a recent cradle model to explain current hyperdiversity. What is significant and interesting about Koenen et al.’s study is the model of high episodic species turnover they propose to explain their results. Under this new model rainforests can be viewed as museums of higher-level taxa and trait diversity that provided the template for recent radiations.